outboxActions [nfc]: Make trySendMessages a thunk action creator.

It'll be easier to write a test for it, when we eventually attempt
that, probably pending resolution of zulip#3881. In particular, without
this change, Flow would complain if we tried to pass
`store.dispatch` -- where `store` is mocked using redux-mock-store
-- as the first argument, for which our custom `Dispatch` type is
expected. I suspect this could be fixed by tightening up the
redux-mock-store libdef's `Dispatch` type. But, for consistency's
sake, `trySendMessages` should probably be a thunk action creator in
any case.

In e5268bb (and confirmed again with logging just now), we
observed that the return value of our function (in this case a
boolean) will be passed through and returned by the `dispatch` call.

src/outbox/outboxActions.js

@@ -62,7 +62,7 @@ export const messageSendComplete = (localMessageId: number): Action => ({
   local_message_id: localMessageId,
 });
 
-export const trySendMessages = (dispatch: Dispatch, getState: GetState): boolean => {
+export const trySendMessages = () => (dispatch: Dispatch, getState: GetState): boolean => {
   const state = getState();
   const auth = getAuth(state);
   const outboxToSend = state.outbox.filter(outbox => !outbox.isSent);
@@ -119,7 +119,7 @@ export const sendOutbox = () => async (dispatch: Dispatch, getState: GetState) =
   }
   dispatch(toggleOutboxSending(true));
   const backoffMachine = new BackoffMachine();
-  while (!trySendMessages(dispatch, getState)) {
+  while (!dispatch(trySendMessages())) {
     await backoffMachine.wait();
   }
   dispatch(toggleOutboxSending(false));